Lab-on-chip devices are becoming increasing explored and used in commercial applications. Typically, these devices are integrated into a microfluidic platform that utilize small volumes of reagents to transport, mix, and perform reactions that used to be performed in larger, bench-top settings. Lab-on-chip devices are known that use droplets as small reaction vessels that contain reagents and/or cells. In typical droplet-based devices, the droplets are formed by a pinching flow of oil around an aqueous phase to generate aqueous droplets or emulsions carried in an oil-based medium. In current droplet-based devices, various pumping devices (e.g., syringe pumps) are also required to pump the aqueous and oil phase components through the device to generate the emulsions. These pumps often require tuning of the flow rates to ensure that droplets of a particular size and composition are formed. Also, the bulky nature of pressure pumps or syringe pumps leads to more complex and larger devices which are less suitable for point-of-care assays as well as difficulty to simply load samples and reagents and mix them in a complete system.
More recently, ferrofluids, or fluids that contain suspended magnetic nanoparticles, have been used in many biomedical applications including various pumping and valving applications. See, Pamme, Magnetism and microfluidics, Lab Chip, 6, 24-38 (2006). For example, ferrofluids have been used as a tool for adjusting the size of droplets when magnetic fluids are applied in a T-junction and flow focusing droplet generators. See, Liu et al., Numerical and experimental investigations of the formation process of ferrofluid droplets, Microfluid Nanofluid, 11, 177-187 (2011). For example, Liu et al. have studied formation of ferrofluid droplets, the velocity field and droplet size in a pressure driven flow focusing device under influence of a uniform magnetic field. See Liu et al., Numerical study of the formation process of ferrofluid droplets, Physics of Fluids, 23, 072008 (2011). Tan et al. have also studied the effect of an external magnet (and also magnetic flux density gradient) and flow rates on droplet size in a pressure driven T-junction droplet generator. See Tan et al., Formation and manipulation of ferrofluid droplets at a microfluidic T-junction, J. Micromech. Microeng. 20, 045004, (2010). However, a need for accurate pumps for droplet generation in these pressure driven systems limits applying these droplet generators as portable devices for point-of-care applications. There is a need for an alternative droplet generating modality that can be utilized to encapsulate reagents and other constituents (e.g., cells) within droplets without the need for accompanying pumping and associated fluidic components associated with traditional droplet-based devices.